The present invention relates generally to fluoroelastomer compositions and, more particularly, to a high solids content, solvated, fluoroelastomer composition which is workable and is useful for both horizontal and vertical applications.
High solids content, solvated, fluoroelastomer compositions are typically used to fill voids, coat, adhere, seal, and protect various substrates from chemical permeation, corrosion, abrasion, etc. Similar low solids content compositions are used as coatings to protect the surface of different substrates. The properties of the sealants, caulks, adhesives, and coatings are commonly adjusted for structural stability and to impart resistance to chemicals, pressure, abrasion, and temperature.
Such compositions often comprise fluoroelastomers which commonly consist of polymers of hexafluoropropylene (HFP), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE). These fluoroelastomer polymers can be cured by a variety of curing agents, for example, amines, bisphenols, and peroxides. Metallic oxides are commonly used to react with HF generated during the curing reaction. In addition, ketones, such as methyl ethyl ketone, and acetates are common solvents used with these fluoroelastomers. One well-known commercial product line comprised of such fluoroelastomers is that referred to as VITON(copyright) fluoroelastomers (a product of DuPont Dow Elastomers L.L.C.). VITON(copyright) fluoroelastomers are comprised of HFP, VF2, and TFE. Another well-known commercial fluoroelastomer, named Fluorel(trademark)/Dyneon(trademark) (a product of Dyneon of Oakdale, Minn.), incorporates similar compounds, such as HFP, VF2, and TFE. Such fluoroelastomer compositions have been used often in molded seal products (sealants), less commonly in coatings, and even less commonly in caulks.
Low molecular weight copolymers of VF2 and HFP display high heat and chemical resistance and can be used as components of seals, wire coatings, and diaphragms in equipment which are exposed to high temperatures and corrosive liquids and gases.
Known high solids content, solvated, fluoroelastomer compositions generally have a solids content of less than 75%. With higher levels of solids, these compositions are not easily manipulable and are unworkable. Such compositions are also prone to cracking, blistering, and bubbling caused by air trapped in the compositions. Lower solids content compositions also typically encounter shrinkage problems when they dry, thus requiring multiple passes. Because of the additional time and expense necessary in using lower solids content compositions, they are less useful.
The present invention comprises a high solids (at least 75% by weight) content fluoroelastomer composition containing HFP, VF2,and/or TFE fluoroelastomers of which at least 40% by weight is vinylidene fluoride. The composition further comprises a solvent, typically an acetate or ketone, and a low solvent absorptive filler (LSA filler). Optionally, the composition may comprise additional additives to effect various properties of the composition, including other fillers. The LSA filler is one which has a combination of properties, such as surface area, particle size, and oil absorption, that, when incorporated into the composition of the present invention, yields a workable, solvated, fluoroelastomer composition.
The composition of the present invention can be used for both horizontal and vertical applications depending on the type and amount of filler and other additives used. Further, in certain embodiments of the invention, the composition includes a low-temperature curing agent containing polyfunctional amines, amino silanes, and ketimines.
Generally, the average Mooney Viscosity of the fluoroelastomer used in the present invention, when measured at 250xc2x0 F., is less than about 60. Mooney Viscosity is calculated as ML 1+10@250xc2x0 F. where xe2x80x9cMLxe2x80x9d represents the minimum torque value for a Mooney Viscometer and the numerical values following xe2x80x9cMLxe2x80x9d represent the amount of time it takes for the sample to heat up (1 minute) and the amount of time the machine runs at temperature (10 minutes), respectively.
The composition may also comprise a thixotropic agent which regulates the shear thinning index of the composition.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.
The present invention comprises a high solids content, solvated, fluoroelastomer composition which is workable and usable in both vertical and horizontal sealing applications to fill voids, coat, adhere, seal, and protect various substrates from chemical permeation, corrosion, abrasion, etc. After placement, the composition hardens to form a seal at the filling site. The hardening may be caused by solvent evaporation, chemical cross-linking of the polymer, or a combination of the two effects. The term xe2x80x9cworkablexe2x80x9d means that it is possible to stir, pour, and apply the product to a wide variety of surfaces without difficulty. For vertical applications, it is important, however, that the flowability of the composition be sufficiently low to prevent spontaneous flow after placement.
The fluoroelastomer component contains hexafluoropropylene (HFP), vinylidene fluoride (VF2), tetrafluoroethylene (TFE), and/or any other fluoroelastomer having suitable properties for incorporation into a high solids composition. Specifically, copolymers of HFP and VF2 or terpolymers of HFP, VF2, and TFE generally form the fluoroelastomer component of the composition, of which preferably at least 40% (more preferably 60%) by weight is vinylidene fluoride. The average Mooney Viscosity (ML 1+10@250xc2x0 F.) of these compounds should be less than about 60, preferably less than about 40, more preferably less than about 10.
Generally, such fluoroelastomers, as are useful in the present invention, will have an average molecular weight at or below 100,000 Daltons.
Fluoroelastomer solutions with lower solution viscosities (usually attributable to a low molecular weight and low compound viscosity of the fluoroelastomers) are prepared so that the compositions of the present invention can be fluid at the desired higher solids content formulations. Table 1 shows the compound and solution viscosities of four fluoroelastomers commonly used in the composition of the present invention, namely Dyneon(trademark) FC 2210X, VITON(copyright) A-100, Dyneon(trademark) FC 2211, and VITON(copyright) A. Each of these fluoroelastomers is a copolymer of HFP and VF2. The compositions are tested for solution viscosity at a 65% solids content in methyl ethyl ketone (MEK) and no filler is included. Fillers would enable compositions of a higher solids content, such as in the present invention.
The solids content of the composition of the present invention is at least about 75% , by weight, preferably from about 80% to about 95% , more preferably from about 80% to about 90%. Solids contents above 95% may also be usable, so long as the molecular weight and the solution viscosity of the fluoroelastomer, the filler content, and other variables are adjusted to maintain the workability of the composition. The high solids content in the composition allows for a reduction in the amount of undesirable volatile organic compounds (VOC""s) produced.
The composition further comprises a solvent component and a filler component. The solvent component may include ketones or acetates, or compounds having similar chemical moieties. The solvent component may include at least one high boiling point solvent, i.e., having a boiling point of at least about 100xc2x0 C., preferably between about 100xc2x0 C. and 200xc2x0 C. High boiling point solvents reduce air trapped in the composition, thereby reducing voids in the composition. They provide better surface quality than solvents with low boiling points. However, use of high boiling point solvents increases the solution viscosity and the drying time of the composition. Examples of such high boiling point solvents are methyl isobutyl ketone, butyl acetate, and butyl cellosolve.
In addition, the solvent component may include a volatile, oxygen-containing, low boiling point solvent, i.e., having a boiling point between about 50xc2x0 C. and 90xc2x0 C. The low boiling point solvents aggressively solvate the compounds of the composition and dry very quickly. However, they are volatile and can cause air entrapment in the composition. Examples of such low boiling point solvents are methyl ethyl ketone and ethyl acetate. Typically, the solvent component comprises from about 25% to about 75% of a low boiling point solvent and from about 25% to about 75% of a high boiling point solvent.
Choosing the appropriate filler as the LSA filler is critical to achieving the low solution viscosities at high solids content required by the present invention. Low solution viscosities for the composition are desirable because it is possible to solvate compositions with low solution viscosities at the desired higher solids levels without increasing solution viscosity to levels that make it difficult to apply the composition. Properties which are important in choosing an appropriate filler include the surface area, particle size, and oil absorption of the filler.
Fillers with large particle sizes will have lower surface areas and tend to absorb less solvent. This type of filler will tend to have lower oil absorption values and create solutions with low viscosities at high solids levels. Variations in particle shape, density, surface activity, etc. will cause variations in these trends, but generally, grades of fillers with larger particles and lower surface areas will be preferable to grades of the same filler with smaller particles and higher surface areas (see Table 3 below). A filler is considered an LSA filler if, when it is incorporated into a fluoroelastomer composition produced by combining 115 grams of filler with 115 grams of VITON(copyright) A fluoroelastomer and combining 112 grams of methyl ethyl ketone (MEK) with 208 grams of the fluoroelastomer/filler mixture (the resulting composition has a solids content of 65%), the fluoroelastomer composition has a solution viscosity of no more than 50,000 centipoises (cps) at 25xc2x0 C. (A 65% solids content was used instead of a solids content at or above 75% because the relative effect of different fillers on solution viscosity is more easily measured at 65% solids).
Various types of fillers that have been found to be useful as LSA fillers provided the surface area of that candidate filler is below the maximum surface area allowed for that filler type. These are listed below.
Note that an Aluminum Silicate filler, having the trade name Burgess 2211 (listed in Table 3), has a surface area below 50m2/g (i.e., 10) which would normally qualify it as an LSA filler. Nevertheless, Burgess 2211 is surface treated by the manufacturer and, as a result, does not have the low solvent absorptive property and is not a usable LSA filler.
In addition to choosing the correct type and grade for the LSA filler, which may comprise one or more fillers (such as from the above list), it is also important to choose the correct amount of such LSA filler. Typically, the weight of the LSA filler component can be up to about 3.5 times the weight of the fluoroelastomer component. Preferably, the weight of the LSA filler is from about 0.5 to about 3.0 times the weight of the fluoroelastomer component, more preferably from about 1.5 to about 2.5 times the weight of the fluoroelastomer component.
Table 2 shows the effect of increasing the amount of a specific LSA filler, nepheline syenite, commercially available as Minex 4 (see Table 3 for characteristics), on the solution viscosity of fluoroelastomer compositions. Specifically, by adding an LSA filler or increasing the amount of an LSA filler (e.g., nepheline syenite Minex 4), the solution viscosity decreases. The filler amounts are expressed as xe2x80x9cphrxe2x80x9d or xe2x80x9cparts filler per hundred parts rubber.xe2x80x9d For example, 100 phr filler would represent an equal amount of filler and fluoroelastomer polymer. For this experiment, the fluoroelastomer used was VITON(copyright) A. The resulting mixture of fluoroelastomers and filler was solvated to a level of 65% solids in MEK (A 65% solids content was used instead of a solids content at or above 75% because the relative effect of different fillers on solution viscosity is more easily measured at 65% solids. The same relative effect of different fillers on solution viscosity, as displayed in Table 2, would occur when testing a composition having a solids content at or above 75%.).
Table 3 shows the surface area, particle size, and oil absorption of various fillers and describes and lists the trade name of each filler. In addition, Table 3 shows the resulting solution viscosity when these fillers are incorporated into a composition produced by combining 115 grams of each listed filler with 115 grams of VITON(copyright) A fluoroelastomer, and combining 112 grams of MEK with 208 grams of the fluoroelastomer/filler mixture. The resulting composition has a solids content of 65% (A 65% solids content was used instead of a solids content at or above 75% because the relative effect of different fillers on solution viscosity is more easily measured at 65% solids. The same relative effect of different fillers on solution viscosity, as displayed in Table 3, would occur when testing a composition having a solids content at or above 75% .).
For horizontal applications, a composition which will flow, level itself, and form a product substantially free of voids caused by air trapped during application is desirable. Its solution viscosity should be from about 5,000 to about 500,000 centipoises, preferably from about 10,000 to about 100,000 centipoises. The low range of viscosities obtained from the fillers in Table 3, such as from about 8,500 to about 50,000 cps, are preferred for horizontal applications.
Table 3 demonstrates the overall relationship between the solution viscosity of a 65% solids fluoroelastomer composition and the properties of a filler incorporated therein. That is, as the surface area and oil absorption of the filler decrease and the particle size of the filler increases, the solution viscosity of the composition generally decreases and the preferred lower solution viscosity values are obtained.
Some of the fillers listed in Table 3, for example, Hi-Sil 233 and Carbon Black N-550, do not qualify as LSA fillers useful by themselves as the required fillers for the compositions of the present invention, but may, in combination with other fillers, which do so qualify, be used to increase the solution viscosity of compositions within the scope of the present invention.
This invention can be used with or without chemical cross-linking agents. If cross-linking agents are desired to increase the strength of the hardened composition, they can be chosen from the amines, peroxides, and bisphenols currently used to cure more commonly produced fluoroelastomers. These cross-linking agents are typically added in amounts from about 0.5% to about 5.0% by weight of the fluoroelastomer component.
Polyfunctional amine curatives (such as methylene diamine, triethylenetetramine, hexamethylene diamine, and other amines with similar functional moieties) offer low temperature cures that are desirable for many applications. These curatives must generally be added immediately prior to application (as a two-part system) to prevent premature curing. Ketimines (which react with moisture to create amines) provide longer pot lives than amines. Amino silanes (such as gamma-aminopropyltriethoxysilane, N-beta-aminoethyl-gamma-aminopropyltrimethoxysilane, and other amino silanes with similar functional moieties) can be effective cross-linking agents and they can also improve adhesion to many substrates.
Bisphenol curatives (such as 2,2-bis(4-hydroxyphenyl) hexafluoropropene) can be incorporated prior to solvation for simple, one-part systems, however, they generally require heat to activate cross-linking. These curatives are often accelerated with a quaternary phosphonium salt, such as triphenylbenzyl phosphonium chloride.
Peroxide curatives (such as 2,5-dimethyl,2,5-di(t-butyl-peroxy) hexane) can also be incorporated into one part systems, but also require heat to activate cross-linking. These curatives are typically used in combination with a co-agent such as triallyl isocyanurate. Peroxides can only be used in combination with polymers that have bromine-modified bond sites.
Optionally, compounds, such as metal oxides, that accelerate curing and increase the cross-link density in the fluoroelastomer polymer by acting as acid acceptors, may be incorporated into the fluoroelastomer composition. The compounds may be incorporated into the composition in a proportion of from about 5% to about 30% by weight of the fluoroelastomer component. Preferred metal oxides for use in the compositions of this invention include magnesium oxide, zinc oxide, lead oxide, and calcium hydroxide.
Further, compositions of the present invention may also include certain other types of additives, such as Theological additives (a thixotropic agent, for example, which provides for non-Newtonian (pseudoplastic) flow), lubricious processing aids, and silicone additives. Examples of such Theological additives include polytetrafluoroethylene (PTFE); aramid fibers; and certain fillers, such as clays, high surface area silicas, high surface area carbon blacks, polyethylene, and other high surface area fillers. For purposes of the present invention, high surface area fillers have a surface area greater than about 50 m2/g. Products produced entirely with the above listed fillers as the only filler would be impossible to work. However, when one of these fillers is present in quantities of about 5-50% of the polymer weight and is added to a composition which also contains an LSA filler (such as those discussed with reference to Table 3), the composition would have desirable flow characteristics for use as a coating, adhesive, sealant, or caulk.
The rheological additives may comprise up to 100% by weight of the fluoroelastomer component in the composition of the present invention. Typical ranges for such additives are 1-4% for aramid fibers and 1-15% for PTFE.
The lubricious processing aids that may be incorporated into the composition of this invention are primarily organic and contain fatty acids; these may comprise waxes, such as carnuba and commercial products such as DuPont Dow Elastomers L.L.C.""s VITON(copyright) Processing Aids (VPA) No. 1, VPA No. 2, Dyneon""s (A 3M Hoechst Enterprise) Dynamar PPA-79, and a fatty oily organic compound (such as TE-88XL paste, a product of Technical Processing of Paterson, N.J.); stearates, such as zinc stearate, potassium stearate, and stearic acid; or oils, which may be incorporated to prevent sticking of the composition to machinery at high temperatures. The processing aid may comprise up to about 10% of the weight of the fluoroelastomer component, preferably about 6%.
The compositions of this invention may also include silicone additives, which reduce the surface tension of the compositions. Low surface tension lessens or minimizes undesirable bubbling in the composition. These silicone additives are typically silicone elastomers or silicone oils, with a low surface tension. A preferred silicone additive is the silicone elastomer GP-30 (a product of Dow Corning). The silicone additive may comprise up to about 10% of the weight of the fluoroelastomer component, but a silicone additive content of about 1% is preferred.
Pigments, such as DISCO 500 (listed in Table 5), carbon blacks, and titanium dioxide, may be added to the composition in order to alter its color. Any suitable pigment compatible with fluoroelastomer compositions may be used.
For vertical applications, a composition of the present invention should have limited or no flow. Such a composition should maintain its shape when placed on a surface, and yet be stirable without requiring significant effort. These properties of the composition can be obtained by use of thixotropic agents, such as polyethylene or aramid fibers, as discussed above, which decrease susceptibility of the composition to flow.
When intended for vertical use, compositions of the present invention should have a shear thinning index greater than about 1.0 and a solution viscosity of about 750,000 to about 6,000,000 centipoises (preferably from about 1,000,000 to about 3,000,000 centipoises with a shear index of at least 1.5). Alternatively, with a lower shear index (about 1), a solution viscosity of about 1,000,000 to about 4,000,000 centipoises is preferred. The shear thinning index (also referred to as xe2x80x9cthixotropic indexxe2x80x9d) is the ratio of the solution viscosity measured at a low speed to the solution viscosity measured at a speed 10 times higher. For example, a composition that has a solution viscosity of 1,500,000 centipoises at 2 RPM and a solution viscosity of 1,000,000 centipoises at 20 RPM would have a shear thinning index of 1.5.
One method of preparing compositions for vertical use comprises modifying the high solids content compositions used for horizontal applications by increasing their solution viscosity. This can be accomplished by increasing the solids content of the composition and/or adding an agent, such as a thixotropic agent, that increases the shear thinning index of the composition to at least about 1.0. These thixotropic agents include inert fibers and other polymers that prevent liquid from flowing freely, such as aramid fibers (Kevlar), PTFE, polyethylene, and polymer microspheres, and fillers, such as high surface area silicas, high surface area calcium carbonate, clays (e.g., aluminum silicate), and high surface area carbon blacks. Further examples of such thixotropic agents are those fillers listed in Table 3 which, when incorporated into the exemplary fluoroelastomer composition on which the data of Table 3 is based, yield a solution viscosity of greater than 2,000,000 cps (i.e., the fillers listed in Table 3 which do not qualify as LSA fillers).
Thixotropic agents provide a structure that resists flow when not contacted by a shearing force, but which is easily displaced by a high shearing force. The particle shape and the particle interaction of particulate materials (e.g., clays) which are xe2x80x9cplatyxe2x80x9d (flaky), are particularly desirable for increasing the shear thinning index of the composition. In fact, particles of such thixotropic agents can flocculate to create strings of particles acting similarly to aramid fibers.